I would not want vinyl beneath the varnish because you may have adhesion problems.  You can put varnish over paint if you rough up the surface a bit first.  Most varnish has a bit of a brown tint so it would change the color of the paint.  

The best/easiest way is to apply the vinyl letering after paint/varnish.  It is cheap and easy to apply.  It will look good for years and is easy to replace when it does start to look shabby.

   Or you could try the "rice paper under epoxy and fiberglass" method, discussed a couple of places in this forum, one of which is here:

https://www.clcboats.com/forum/clcforum/thread/52621.html

This is the approach used by surfboard makers - lots of stuff on youtube about it.

Mine turned out nicely - I'll see if I can figure out how to post a picture!

   Thx for the suggestions, I am intrigued by the rice paper idea, will do some more reserach on that.  Will also keeep the vinyl lettering option in my back pocket in case nothing else works out.  I have ordered those for my fiberglass sailboat in the past and know what's involved there.

I ordered lettering from drytransfer.com You supply them the art - lettering or graphics, it doesn't matter. I sent them a pdf in the font I wanted, generated from MS Word. Then you specify size and color. In a few days I received a dry transfer that you rub onto your hull. I applied a single coat of varnish with minimal sanding over the epoxy before putting on the transfer. Then seven coats of varnish over that. Worked like a charm.